This invention relates to a thermal transfer recording sheet for conducting thermal transfer recording by utilizing a thermal head or the like and a method for the thermal transfer recording.
Well known, conventional thermal transfer recording sheets having a melt transfer temperature of about 60.degree. C. are produced by forming a thermal transfer recording layer of about 4 .mu.m in thickness composed of only a recording material prepared by using, as a binder material, a hot-melt material consisting of 20% by weight of carnauba wax, 40% by weight of ester wax, 10% by weight of mineral oil and 10% by weight of other adjuvants. This hot-melt binder material is mixed with 20% by weight of a pigment coloring material, on the surface of a sheet-like heat-resistant substrate such as polyethylene terephthalate (PET) film, condenser paper or the like having a thickness of about 7 .mu.m, by a hot-melt coating method (see, for example, Y. Tokunaga and K. Sugiyama, "Thermal Ink-Transfer Imaging", IEEE Trans. on Electron Devices, vol. ED-27, pp. 218-222, 1980.).
A thermal transfer recording sheet produced by forming, on the surface of a sheet-like heat-resistant substrate such as polyethylene terephthalate (PET) film, condenser paper or the like having a thickness of 9 .mu.m, a thermal transfer recording layer having an uneven surface by mixing a recording material which contains a binder material and a coloring material and whose viscosity is lowered and controlled by temperature-raise recording control with ink transfer helping particles having a melting or softening point higher than that of the aforesaid binder material and a particle size larger than the thickness of a layer made of the recording material is disclosed in T. Kohashi, H. Onishi and H. Esaki, Japanese Patent Application No. 227155/84 which has not yet been published.
Thermal transfer, using these thermal transfer recording sheets, is conducted, in general, by pressing the thermal transfer recording sheet on a recording medium (an image-receiving sheet) such as recording paper or the like, pressing a well-known thermal recording head on the reverse side of the sheet-like heat-resistant substrate, selectively subjecting the thermal transfer recording layer to temperature-raise recording control through the substrate sheet by means of the thermal recording head, and thereby melt-transferring an ink material to the recording medium.
In the case of a thermal transfer recording sheet produced by placing one or more thermal transfer recording layers composed of a recording material alone on the surface of a sheet-like heat-resistant substrate, the recording material is adhered and transferred to a recording medium only after melting of the binder material is completed from the sheet-like heat-resistant substrate side to the thermal transfer recording layer surface. In this case, the recording material melted in the direction of the thickness of the thermal transfer recording layer is adhered and transferred at once to the recording medium, and hence such a thermal transfer recording sheet is useful for binary density recording such as letters, figures and the like.
In this case, when attempts to transfer the method recording material to the same recording medium always in the same amount according to heat energy supplied by temperature-raise recording control, without causing unevenness of transfer is tried, the transfer is seriously influenced by unevenness of the contact between the recording medium and the thermal transfer recording layer. Since the recording material is melted and has a lowered viscosity, the unevenness of transfer is influenced mainly by the condition of surface of the recording medium on which transfer recording is conducted and by a definite pushing pressure at the time of the contact. However, when a thermal head is used for the temperature-raise recording control, the pushing pressure is preferably 2.0 kg/cm.sup.2 or less because of a problem of the physical strength of the thermal head.
In the case of a thermal transfer recording sheet produced by placing one or more thermal transfer recording layers having an uneven surface formed by incorporating ink transfer helping particles into a recording material, on the surface of a sheet-like heat-resistant substrate, heat energy is supplied from the sheet-like heat-resistant substrate side, resulting in a lowering of the viscosity of the recording material in the sheet surface portion and in production of a recording material having a lowered viscosity on the surface of the ink transfer helping particles by heat conduction. These recording materials having a lowered viscosity penetrate along the surfaces of the ink transfer helping particles owing to their thermal expansion caused by the lowering of the viscosity and to capillarity between the ink transfer helping particles and the recording medium surface. They also adhere to the ink transfer helping particles and are transferred to a recording medium together with the ink transfer helping particles. Therefore, the amount of the transfer is continuous according to the degree of lowering of the viscosity of the recording material, namely, the heat energy supplied by temperature-raise recording control. Accordingly, continuous analogue gradation recording can be conducted by temperature-raise recording control. However, in this case, when attempts are made to transfer the recording material to the same recording medium always in the same amount according to heat energy supplied by temperature-raise recording control, without causing unevenness of transfer (including unevenness of recording density), the transfer is seriously influenced by unevenness of contact between the recording medium and the thermal transfer recording layer. In this case when particle size distribution of the ink transfer helping particles is constant, the unevenness of transfer is influenced mainly by the condition of surface of the recording medium on which transfer recording is conducted and by a definite pushing pressure at the time of the contact. However, also in this case, when a thermal head is used for the temperature-raise recording control, the definite pushing pressure is preferably 2.0 kg/cm.sup.2 or less because of a problem of a physical strength of the thermal head.
Therefore, when these thermal transfer recording sheets are used, a surface-treated sheet such as calendered paper, coated paper, synthetic paper or the like should heretofore be used as a recording medium. In the case of paper for general use having a low surface smoothness, for example, a Beck smoothness of 500 seconds or less, the transfer is too uneven for many reasons, such as no transfer of a recording material to be transferred, etc.
Further, in the case of thermal transfer recording on previously conducted thermal transfer recording, such as color repetitive recording and the like, by use of these thermal transfer recording sheets, particularly when the same kind of the recording materials is used, the transfer is generally easier than transfer on the surface of a recording medium. Even if the surface smoothness of the recording medium is improved, intermediate tone and color reproducibility are deteriorated by excessive transfer of the recording material at the time of repetitive recording, so that unevenness of transfer is observed.
The quality of the resulting record is greatly lowered by these uneven qualities of transfer.